docs: def _precesion(date):
      """Precession in degrees
      """
  
      t = date.change_scale('TT').julian_century
  
      zeta = (2306.2181 * t + 0.30188 * t ** 2 + 0.017998 * t ** 3) / 3600.
      theta = (2004.3109 * t - 0.42665 * t ** 2 - 0.041833 * t ** 3) / 3600.
      z = (2306.2181 * t + 1.09468 * t ** 2 + 0.018203 * t ** 3) / 3600.
  
      # print("zeta = {}\ntheta = {}\nz = {}\n".format(zeta, theta, z))
      return zeta, theta, z